The present application is directed to contact electrography, and more particularly to an addressable imaging belt configuration for use in a contact electrographic system.
Xerography, also referred to as electro-photography, can be broken down into seven basic steps: (i) Charging of a photoconductive drum or belt with a scorotron; (ii) Latent image formation by image wise discharge using a raster optical scanner (ROS) or LED array; (iii) Development of toner (either two component or monocomponent) supplied from a donor roll; (iv) Electrostatic toner transfer to an intermediate belt; (v) Transfer from the intermediate belt to paper; (vi) Fusing of the toner onto the paper under high temperature and pressure; and (vii) Cleaning and erasing of the photoreceptor and intermediate transfer belts.
At the low end of the digital printing market, traditional xerography is being threatened by much simpler lower cost marking technologies. For example, in the small office/home office (SOHO) market, printing is dominated by lower cost ink jet approaches. In the high end commercial printing market, it is difficult for xerography to address the substrate latitude and wide media format that quick turn computer to press offset lithography systems can offer. In addition, factoring in service and consumable expenses, quick turn lithography presses have a lower cost structure for run lengths as short as 500 pages.
An advantage xerography still maintains is the ability to print a full page of variable data at higher speeds than drop on demand ink jet printing. Thus a means for reducing the complexity of xerography while increasing substrate latitude and media format in a cost effective manner has the potential to increase the market share for xerographic printing.
One long standing idea for simplifying xerographic printing is to use a direct write concept known as contact electrography. FIG. 1 illustrates a conceptual view of a contact electrographic system 10 which includes a write head array 12, having a series of closely spaced cantilevers 14 to directly address a surface 16 of a dielectric imaging drum 18. This process is used to form an electrostatic image onto the dielectric imaging drum 18 by making direct contact to the surface and of the imaging drum 18. Thus contact electrography can eliminate the need for the ROS optical subsystem and associated subtle print artifacts.
Here, an image-wise charge pattern is formed onto a retaining dielectric drum using a write head containing an array of electrode elements in contact with the drum. Imaging is then accomplished by selectively applying a high voltage to the electrodes to induce charge onto the drum surface or by selectively applying a grounding potential to erase charge from this surface. Additionally, a common potential can be applied to all electrodes and then such electrodes can be made to selectively bend further and thereby selectively touch the charge retaining surface. While these type of contact electrography reduces front end complexity, it has suffered from other imaging problems including but not limited to: (i) Non-uniformity of the charge written into a dielectric by the electrode arrays; (ii) Non-repeatable dielectric charging due to variations in contact pressure (iii) Ghosting caused by not being able to fully erase trapped charges; (iv) Reduced signal-to-noise (S/N) development due to triboelectric noise and low voltage requirements imposed by lateral air breakdown limitations between nearest neighbor electrodes; and (v) Contamination of the write electrode array ahead from debris and residual toner.
(i-iii) Contact Charging Uniformity, Repeatability, and Ghosting Issues
Uniformity is an issue that plagues any printing technology that relies on an array of elements to write either a latent electrostatic image or a directly marked image on paper. The need to tune the performance of individual writing elements, calibrate their performance over temperature, or build in redundancy for dead elements dramatically adds to the overall cost. In addition, the need for adding circuits that can address these elements can also be complex and costly.
Uniformity issues in contact electrography arise from variations in contact pressure and tip geometry. These issues are compounded by vibrations of the drum which change the relative pressure onto the dielectric and by non-uniformly wear of the tip shape over time. These phenomenon lead to changes in stored charge which can lead to toner development curve shifts, mottle, and banding. In addition to these serious issues, there are mottle issues related to tribo-charging from the friction between the write electrodes and the dielectric. Typical variations in charge densities of only a few percent can lead to observable fluctuations in toner pile height and mottle.
To eliminate such problems a concept as disclosed in U.S. Pat. No. 6,362,845, entitled “Method and Apparatus for Electrostostatographic Printing Utilizing an Electrode Array and a Charge Retentive Imaging Member,” by Genovese, Issued Mar. 26, 2002, hereby incorporated by reference in its entirety, and illustrated in FIG. 2, teaches a contact electrographic system 20 where contact between a write head print array 22 and an imaging belt (or imaging drum) 24 is through use of uniform metal islands 26 lithographically defined and patterned on the top, upper or imaging surface of the imaging belt (or imaging drum) 24.
In this approach, the amount of charge stored is not varied due to subtle differences in the electrode shape or pressure of the electrode tip on the metal island surface because the charge stored is determined only by the applied voltage and the capacitance of the metal island to a ground plane underneath. Previously written charge can easily be extracted from the metal islands by applying zero volts to the write electrode thus avoiding latent image ghosting. This is not the case for dielectric films because the charge can be immobilized due to deep charge traps in the insulating dielectric.
For the case where charge is deposited into an array of metal islands 26, the capacitance of an individual island is only on the order of 1 femtofarad. The RC time constant associated with direct charge injecting into an island is negligible compared to the RC time constant associated with parasitic capacitance of the electrode fingers. As long as the contact resistance to the islands is relatively low (<<KΩ) as would be the case for metal tip to metal island contact, the slew rate of the high voltage electronics is likely to be the time limiting step for writing. For example a page width addressable array built on glass, amorphous silicon high voltage (HV) transistors typically will not work faster than 100 kHz. Thus the total time for injecting charge can be consider to be on the order of 10 uS. This is more than adequate to print an entire 8½″×11″ page at more than 500 ppm.
Another approach to creating charge storing topside metal islands include the use of randomly scattered metal particles embedded within a dielectric layer. Such an approach assumes the global dispersion of metal islands within a dielectric is such that islands do not come too close together so as to avoid shorting of adjacent writing electrodes and that the global uniformity of the dispersion leads to uniform prints. Such an approach also assumes that each electrode needs to encompass roughly the same touch area such that image uniformity is preserved. The advantage of this method is no lithography need be done in the manufacturing of the latent image carrier.
(iv) Low Voltage Development
Another issue with contact electrography is the need for a development system that works at voltages below the breakdown strength of air. This is not a problem for liquid toner systems which can operate well below 100V but the use of liquid toner is not desirable in the home or in the office. Most dry toner systems use two component magnetic brush development technologies requiring 500-600V difference between the imaging and non-imaging areas. Unfortunately, at such high voltages breakdown can occur in the air region just above the surface between adjacent metal islands or adjacent stylus tips. Such breakdown can lead to an increase in tip wear. Typically, the voltage applied cannot exceed around 400V before some form of lateral breakdown is observed. Therefore, a lower voltage development system needs to be used. FIG. 3 depicts an example of two toner development curves wherein one curve 33 represents a highly conductive two-component development magnetic brush system (CMB) and one line curve 34 represents a more typical semiconducting two component toner development system. FIG. 3 shows the CMB system can be optimized to perform well at only a 300V contrast potential difference between imaging and non-imaging areas. More particularly, FIG. 3 graphically illustrates the developed mass per unit area of toner is increased the more conductive a two component magnetic brush system becomes. Dotted line 32 shows the approximate layer thickness necessary to achieve full solid area coverage (1.5 monolayers) of EA toner. A conductive magnetic brush (CMB) system 33 has roughly twice the development efficiency as a semiconducting development curve 34 with the trade off that the CMB development curve has roughly twice the slope and thus there is increases sensitivity to small variations in the image potential of the latent imaging surface.
However the problem with using such a CMB development system together with a direct write architecture is that when the conductive development brush touches a conductive metal island it will electrically short the stored charge on the island. Thus the islands must somehow be shielded from direct contact with a CMB system but be accessible to contact electrostatic delivery of charge at the same time.
(v) Contamination Issues
Another problem for the direct contact approach is contamination. In a real system the latent imaging surface will come into contact will all sorts of debris and varying environmental conditions. A simple calculation shows that for an 8½″×11″ page with 50% toner coverage, assuming roughly an average toner particle diameter of 5 microns, the number of toner particles printed on a single page is approximately 1 billion. Cleaning systems will remove most but not all of the residual toner left behind. This concept is illustrated in FIG. 4, which is a diagram of a contact electrographic system 40 including a latent image drum 42, having charged applied by a direct write head electrode array 44. As the charged latent image drum 42 rotates, a development roller 46 applies toner which is transferred to an image transfer drum 48, and then onto paper or other substrate 50. A cleaning brush 52 is used to remove residual toner 54 prior to the drum being re-written. As FIG. 4 illustrates, it is possible for some of this residual toner 54 to be missed by the cleaning brush 52. This missed residual toner 54 can become trapped underneath one of the electrodes on the direct write electrode array 44. Additionally, small (e.g., as small as micron sized) paper fibers can migrate through the system even though paper is never brought into direct contact with the latent imaging surface.
Unfortunately, a single toner particle trapped between a write electrode and the imaging surface could increase the contact resistance substantially above 100KΩ. Given a parallel parasitic capacitance of a write electrode finger could be as high as 1 nF, this RC time constant combination would then start to prohibit sufficient island charging at normal line printing speeds in the range of 4 kHz per line and lead to an unacceptable line defect across an entire print. In addition, the associated electrode abrasion from trapped toner debris could lead to the further spreading of surface contamination and lead to changes in imaging surface electrical leakage over time. These reliability issues pose a large hurdle to the practical implementation of contact electrography.